The treatments necessary for the manufacture of semiconductor integrated circuits utilize for a large part processes of depositing or etching carried out in a treatment gas or of a mixture of several gases under reduced pressure. Besides the process of low-pressure chemical vapour deposition, known under the designation LPCVD, in which a chemical reaction is obtained merely due to the high temperature to which the substrate is brought, other processes more frequently give rise to an activation of the treatment gas by a plasma formed in the vacuum chamber, in which processes the substrate, in electrical connection with its support, forms one of the electrodes, while another electrode is disposed parallel to the substrate at a given distance therefrom.
Depending upon the type of process to be used, the power of the electrical field applied to the electrodes for producing the plasma can be greatly different, while the temperature at which the substrate must be kept can also vary within wide limits. As a result, in general a heat flow must be provided between the substrate and its support in one or the other direction, that is to say to that the substrate when the losses by radiation are preponderant, or to cool it when the power used in the plasma is high and exceeds the radiation losses.
Finally, annealing treatments at given temperatures in accordance with a very rapid cycle (less than one minute) are also susceptible to being carried out in a vacuum chamber, in which a atmosphere of a neutral (or reducing) gas may be established under reduced pressure in a chamber, or in a high-vacuum. Such rapid annealing treatments are used inter alia to obtain micro-alloys between materials of different kinds, which are initially superimposed.
It is well known that it is difficult to obtain a satisfactory heat exchange between a substrate and its support when the assembly is arranged in a chamber under reduced pressure. Now, this heat exchange plays an essential part in the control of the temperature to which the substrate must be brought during the treatment to obtain a temperature homogeneity of the substrate as satisfactory as possible and to obtain, if necessary, a speed of temperature increase of the substrate and a speed of cooling as high as possible. The processes of treatment in a vacuum or in a partial vacuum are in fact mostly very sensitive to the temperature so that a poor control of the temperature of the substrate would involve an unacceptable dispersion of the results both between one operation and the other and as a function of the position on the surface of the same substrate.
In order to meet these difficulties, it has already been proposed to inject between the back surface of the substrate and the supporting surface of the support a gas thus forming a heat-exchanging gas cushion therebetween facilitating the heat exchange between the substrate and the supporting surface of the support, whose temperature can be regulated in an active manner.
An apparatus utilizing such a technique and corresponding to the definition given in the introductory paragraph is known from the document German application No. DE-A-36 33 386.
In the known apparatus the gas injected under the back surface of the substrate flows along this surface and escapes at its periphery, at which it then reaches the atmosphere of the vacuum chamber. A flow of He or the treatment gas is used to obtain the heat exchange between the substrate and the support.
The fact that, in order to obtain the thermal contact of the substrate, the same gas is used as that by which the reaction provided in the treatment is fed, imposes serious problems.
In the first place, the reaction gas is not always the most effective with respect to the thermal conduction and it would be very desirable that the gas of one's own choice can be used, independently of the treatment gas. In the second place, the escape into the space of the gas having served for the heat exchange between the carrier and the substrate will influence the pressure in the vacuum chamber in the proximity of the periphery of the substrate.
If another gas than the treatment gas should be used for obtaining the thermal conduction between the substrate and its support, a difficulty could be met due to the inhomogeneity of the gas in the chamber because the gas injected under the substrate is finally mixed with the gas in the chamber at the periphery of the substrate. It would be possible to reduce this effect by considerably reducing the flow of gas injected under the substrate, but this measure again leads to a limitation with regard to the possibilities of heat exchange between the substrate and the support.
Otherwise, when using the same gas as the treatment gas and when the latter is susceptible to being decomposed under the influence of an increase in temperature, this decomposition starts at the passage of this gas through the support. In this case, the result consists in undesirable deposits within the support, at its surface and at the back surface of the substrate.